The efficacy of a medical diagnostic or therapeutic procedure employing a chemical agent is often dependent upon achieving an effective concentration of the chemical agent in the subject. I.e., a more intense effect is frequently exhibited by a higher dose (i.e., by providing a higher concentration of the agent in the subject's body), at least up to a limit usually dictated by toxicological concerns and/or undesirable side effects.
Many diagnostic and chemical agents, particularly diagnostic agents, have molecular structures that comprise an "active group" (i.e., a chemical moiety that is directly responsible for the desired diagnostic or chemotherapeutic effect) connected to other molecular structure(s) useful for any of a variety of purposes such as, but not limited to, solubility of the agent, absorption of the agent, physiological transport of the agent (such as through biological membranes), biotransformation of the agent, or targeting of the agent to a particular situs in the subject. The vast majority of such agents have only one active group per molecule of the agent.
Administering higher doses of conventional agents to achieve maximal therapeutic or diagnostic effect may not always be possible because of various undesirable dose-related effects. In certain instances these effects are related simply to the number of molecules of the agent present in the subject's body. If it were possible to simply reduce the number of molecules without decreasing the effect, many such problems could be either eliminated or substantially reduced.
As an example of a diagnostic technique that typically employs a chemical agent as described above is "Magnetic Resonance Imaging" (MRI). This technique employs the general principles of Nuclear Magnetic Resonance (NMR). NMR is based on the behavior of atomic nuclei that have non-zero nuclear spins (i.e., I not equal to zero), e.g., .sup.1 H, .sup.13 C, and .sup.31 p. When such nuclei are placed in an externally applied magnetic field, their rotations about their respective internal axes (i.e., their "spins") cause them to precess at a particular frequency in the external field.
MRI images are obtained by placing a subject in an external magnetic field and detecting the effect on nuclear spins as the external field is manipulated. Manipulation of the external field is usually performed using pulsed radio-frequency (RF) energy. The RF energy is at the precession frequency of the targeted nuclei. As a result, certain nuclei absorb the energy. At the end of an RF pulse, the precessing nuclei emit the absorbed energy as they relax back to equilibrium. The emitted energy is received by the RF coils used for image formation.
The time required for the nuclei to relax after an RF pulse ends is measured. This time is profoundly affected by the immediate chemical surroundings of each emitting nucleus. For example, hydrogen nuclei associated with fats have substantially different relaxation characteristics compared with hydrogen nuclei associated with water.
MRI images reflect certain intrinsic variables associated with nuclear spins within tissues. One intrinsic variable is termed the longitudinal, or T1, relaxation. Another is the transverse, or T2, relaxation. T1 and T2 relaxations occur over discrete amounts of time that can be deliberately manipulated.
The contrast of MRI images can be substantially enhanced by using contrast-enhancing agents. Certain of these agents produce marked shortening of the T1 relaxation time in the tissues where the agents can localize in sufficient concentrations. Such shortening of the T1 relation time produces high signal on T1-weighted images. Other agents can affect the T2 relaxation time, or both the T1 and T2 relaxation times.
The only contrast-enhancing agent enjoying substantial clinical use is gadolinium-DTPA, a type of gadolinium chelator. Gadolinium is particularly favored because it has seven unpaired electrons that produce an especially strong paramagnetic effect on adjacent water protons, which causes marked T1 relaxation acceleration (i.e., shortening of T1 relaxation time). Since paramagnetic metal ions useful for relaxivity enhancement are usually toxic, placing such ions in physiologically compatible complexes reduces their toxicity without substantially reducing their effectiveness.
Certain compounds termed nitroxides are also receiving considerable attention as MRI contrast-enchancing agents. Nitroxides are among few examples of organic paramagnetic compounds. Generally, organic compounds have closed electron shells in which all the electrons are paired; such compounds are generally termed "diamagnetic." Only compounds having unpaired electrons can be paramagnetic; such compounds, also termed "free radicals," are usually highly reactive and thus normally cannot be isolated. Nitroxides, also termed "nitroxide free radicals," are unusual organic free radicals because many nitroxides can be synthesized, handled, and utilized as conventional organic compounds. However, due to the presence of at least one unpaired electron in each nitroxide compound, these compounds can act as MRI contrast enhancers.
As with most other chemotherapeutic and diagnostic agents, as discussed above, conventional MRI contrast-enhancing agents have only one chelator or nitroxide group per molecule. These agents are typically short-lived in the subject's body or other physiological environments. Thus, in many instances, large doses must be administered in order to achieve a desired degree of contrast enhancement. In other instances, maximal contrast enhancement cannot be achieved without administering a potentially fatal or otherwise physiologically intolerable dose to the subject. Another problem is that nitroxides tend to be rapidly reduced in the body. Heretofore, reduction problems have been addressed by administering large amounts of the agent to the subject with the intent of "swamping" the reduction reaction. Unfortunately, such large doses of nitroxides can be toxic and/or cause osmotic disequilibria in the body.
In J. F. W. Keana's U.S. Pat. Nos. 5,135,737 and 5,252,317, incorporated herein by reference, certain molecules termed "amplifiers" or "amplifier molecules" are described. Each molecule of such amplifier molecules has multiple diagnostically or therapeutically active groups (such as, but not limited to, nitroxides or paramagnetic metal-ion chelators). Thus, administering a particular number of molecules of such amplifiers results in a more enhanced effect than administering an equal number of conventional molecules having only one active group per molecule. Also, fewer individual "particles" need be administered to achieve an acceptable effect when amplifiers are used. This is important in the control of the osmolarity of an administered solution of the agent. More particles can result in a greater imbalance in osmolarity and thus greater pain sensation during administration of the compound. Because amplifiers used for contrast enhancement are generally larger than conventional molecules, amplifiers have a slower, more optimal "tumbling rate" which leads to greater enhancement per paramagnetic center.
Despite the foregoing, there is an ongoing need, and thus an ongoing effort to find, other amplifiers having optimal properties of maximal contrast enhancement per mole and lowest possible toxicity.